Electronic counter-countermeasures system for employment against enemy jamming

ABSTRACT

The present invention relates to a counter-counter-measures system designed to reduce the effectiveness of a so-called &#39;&#39;&#39;&#39;jamming&#39;&#39;&#39;&#39; signal transmitted by an airborne enemy target in order to decoy a missile launched from an aircraft with the intention of impacting such target. The invention is intended to be employed in conjunction with apparatus on the missile for tracking the target by receiving the doppler frequency wave therefrom. The essence of the concept is the provision of means for discriminating between this true doppler signal from the target and a false doppler signal generated by the target and transmitted therefrom on a frequency which differs from the basic doppler frequency. Included in the invention apparatus is means which refuses to respond to this &#39;&#39;&#39;&#39;false&#39;&#39;&#39;&#39; doppler signal and instead &#39;&#39;&#39;&#39;holds over&#39;&#39;&#39;&#39; the true doppler signal so that the missile will continue to be guided toward the target regardless of the &#39;&#39;&#39;&#39;jamming&#39;&#39;&#39;&#39; energy. In other words, the missile tracking system is conditioned to respond only to the &#39;&#39;&#39;&#39;true&#39;&#39;&#39;&#39; doppler signal.

United States Patent 1 1 Foin, Jr. et al.

[ 1 Aug. 19, 1975 i 1 ELECTRONIC COUNTER-COUNTERMEASURES SYSTEM FOR EMPLOYMENT AGAINST ENEMY JAMMING [75] inventors: Owen F. Foin, ,Ir., Fresno; Frank P.

Miley, Camarillo, both of Calif.

[73] Assignee: The United States of America as represented by the Secretary of the Navy, Washington, DC.

122 Filed: Aug. 5, I965 1211 Appl. No.: 477,620

521 US. Cl 343/7 A; 343/8; 343/18 E 1511 lm. cm G015 9/46; HO4K 3/00 [58] Field of Search 343/5. 7, s, 9. 17.1, 18,

Primary ExaminerMalcolm F. Hubler Attorney, Agent, or Firm-Richard S. Sciascia; Joseph M. St. Amand [57] ABSTRACT The present invention relates to a counter-countermeasures system designed to reduce the effectiveness of a socalled jamming signal transmitted by an airborne enemy target in order to decoy a missile launched from an aircraft with the intention of im pacting such target. The invention is intended to be employed in conjunction with apparatus on the missile for tracking the target by receiving the doppler frequency wave therefrom. The essence of the concept is the provision of means for discriminating between this true doppler signal from the target and a false doppler signal generated by the target and transmitted therefrom on a frequency which differs from the basic doppler frequency. included in the invention apparatus is means which refuses to respond to this false doppler signal and instead holds over" the true doppler signal so that the missile will continue to be guided toward the target regardless of the jamming" energy. In other words, the missile tracking system is conditioned to respond only to the true" doppler signal.

7 Claims, 5 Drawing Figures l l IXER V E V DlSCRlaltL/ll iiATOR GATE i. i R 7 ll 1| 1 (R) A 42 ii LOCAL -28 MEMORY 08C. ClRCUlT l D SCRLlgLNA I OR GATE F, g 3 r k B11 OUTPUT GATE "All GATE "All (R) MEMORY CIRCUIT DISCRIMIINATOR LU 1 [IF 2 TARGET SEEKING MISSILE DISCR\MINATOR AMPLIFIER MIXER LOCAL OSC.

PATENTED AUG 1 91975 Fig. 3

INVENTORS OWEN F. FOIN JR.

TARGET JAMMING TRANSMITTER FRANK P. MILEY AMMING IGNAL 22 DOPPLER RETURN FROM TARGET I i 1 i f +Af MISSILE- LAUNCHING AIRCRAFT PATENIEII AUG I 9 I975 SIZU 2 BF 2 INPUT V L'AJ GATE OUTPUT AMPLIFIER "A" (RI 42 30 II I I 32 36\ as 44 Augmnc MEMORY CONTROL 54 40 l DISCRIIIIBIILNATOR GATE IIBII F I g. 4 B

INPUT L F. DISCRI'IMIINATOR V GATE OuTI T AMPLIFIER I 30/ R) 42 I I I 36 k 32 I 38 AUTG%|\/|T|C MEMORY 44 CONTROL CIRCUIT i: CONTROL CIRCUIT II I OISOR I E NATOR GATE II II a B X F! g. 5 B FRANKPMILEY A r mR/vEY ELECTRONIC COUNTER-COUNTERMEASURES SYSTEM FOR EMPLOYMENT AGAINST ENEMY JAMMTNG The invention described herein may be manufac tured and used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor.

The present invention relates in general to signalprocessing apparatus such, for example, as that used on a missile or other projectile which has been launched from an airborne vehicle toward a target and where such target is equipped to generate and transmit a socalled jamming signal the purpose of which is to decoy such missile from its intended objective.

The invention herein described is specifically but not exclusively designed for application to airborne radar systems wherein a Doppler frequency component of the return or radar echo signal is employed to guide a missile (or other projectile) toward a target by enabling the missile to track the target during at least the terminal portion of missile flight. This return, or radar echo, signal which contains the Doppler information is generally a microwave radio-frequency signal having a frequency somewhat higher or lower than that of the transmitted signal.

In order that such radar systems may be effective, they must automatically track or follow changes in the quantity producing the Doppler information to yield an output signal representing that quantity. Since the magnitude of the quantity is represented by the Doppler information. such radar systems include frequency trackers as one of their most important instrumentalities. This frequency tracker comprises a search circuit which, on initial operation, continuously scans or searches the Doppler frequency band for a Doppler return signal. When the signal is found, the frequency tracker locks onto it and thereafter stops scanning and follows the Doppler signal.

It is possible to cause such radar systems to malfunction by deliberately subjecting them to microwave radiation from an external transmitter such as might be carried, for example, on a target toward which the missile has been launched. This radiation from the target. when transmitted on the same wavelength on which the radar is operating, has the effect of augmenting the echo signals. The target transmitter in such an environment would be modulated at a frequency which would initially be identical to the Doppler frequency but which would slowly vary from this quantity so as to result in a false" Doppler signal being transmitted to the missile. This false Doppler signal will be picked up by the missile and the tracking apparatus thereon will follow such false Doppler signal rather than the true Doppler signal generated by echos or reflections from the target of the energy transmitted by the launching aircraft. in other words, the tracking apparatus of the missile will latch on to a signal which is not truely indica tive of the position of the target. Consequently, the missile will seek out a point in space other than that at which the target is actually located.

Countermeasures systems of the above type are designated as deceptive since they tend to draw the this sile away from the target and thus mislead the missiles tracking apparatus as to the true location of the object the destruction of which is desired. Various expedients have been proposed for overcoming this type of countermeasures, but none of these has proven to be fully satisfactory. This is principally due to the severe requirements placed upon such apparatus. especially with respect to the speed at which performance must be carried out and the built-in reliability which they must possessv Furthermore, all such countercountermeasures systems must be fully automatic in action. and be sufficiently effective to justify their use under extremely adverse conditions.

In accordance with a specific application of the present invention, an apparatus is provided for use on a missile or other projectile launched with the intention of impacting a target on which is located a transmitter capable ofcmitting ajamming signal. The invention apparatus is so designed as to incorporate means for tracking a target by receiving a Doppler frequency wave therefrom in the usual fashion. However, this apparatus contains means for discriminating between this true Doppler signal from the target and a false Doppler signal transmitted therefrom on a frequency which differs from the basic Doppler frequency and has been generated by the target with the express intention of pulling the missiles tracking apparatus away from the true target and causing the missile to track a spurious point in space. More specifically, the ECCM (electronic countencountermeasures) apparatus on the missile is designed to incorporate therein a so-called speed-gate system arranged to ordinarily receive the true Doppler signal reflected from the target. However, when the target emits a false Doppler signal, the circuitry on the missile will not respond thereto, but will hold over" the true Doppler signal so that the missile will continue to be guided toward the target regardless of the jamming energy transmitted therefrom. In other words, the missile is conditioned to respond only to the true Doppler signal, and any information received therefrom on a different frequency will not pass through the demodulating circuit and affect the tracking apparatus. Hence, the target cannot avoid destruction by decoying the missile to a point in space other than that at which the target is actually located.

One object of the present invention, therefore. is to provide a circuit which is intended to be responsive to a given rate of change of an electrical quantity, and which continues to be responsive to this rate of change even though an input signal is applied thereto which changes at a rate other than that of the original signal.

A further object of the invention is to provide an electronic counter-countermeasures system particularly useful in deceptive types of countermeasures.

A still further object of the invention is to provide a system, operating in a countermeasures environment, which enables a missile launched from a missilecarrying aircraft to track a target and impact the latter regardless of any electronic countermeasures which the latter may employ to avoid destruction.

An additional object of the invention is to provide an electronic counter-countermeasures system which locks on to a Doppler signal from a target and remains locked on to such target even though the latter may transmit a Doppler signal at a different frequency to the missile in an attempt to decoy the latter from its in tended objective.

Other objects, advantages and novel features of the invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings wherein;

FIG. 5 is a block diagram of a modification of the apparatus of FIG. 4 showing a variable control unit for one of the component parts of the electrical network.

Referring now to the drawings, there is shown in FIG. I a missile or other target-seeking projectile which has been launched from an aircraft [2 toward an airborne target 14. The missile-launching aircraft 12 carries thereon a microwave transmitter which is designed to radiate a CW signal I6 of frequency f toward the target I4, a portion of this energy then being reflected from the target to the missile l0. Ordinarily, this reflected signal 18 received by the missile 10 contains a Doppler component Af, which varies at a rate which is indicative of the relative change in position of the missile I0 and target I4. This rate of change of the Doppler signal is employed in a known manner to guide the missile 10 toward the target 14 during at least the terminal portion of missile flight.

In describing the operation of the present invention, it will be assumed that the target 14 is equipped with a jamming transmitter which is capable of receiving the signal 16 transmitted by the missile-launching aircraft l2 and of adding thereto a component A f which is variable in frequency from a condition of substantial equality to the Doppler signal A f. to a further frequency which is different and distinct from the original Doppler frequency A f Such jamming transmitters are well known in the art, and details thereof will not be described in the present application. The present invention relates to apparatus carried by the missile l0 and which is designed to pick up the microwave energy reflected from the target 14 and to utilize such reflected energy to control the trajectory of the missile so that it will eventually impact the target and bring about a destruction of the latter.

Before proceeding with a description of this apparatus carried by the target-seeking missile I0, it might be desirable to point out the manner in which the jamming transmitter carried by the target 14 can effectively decoy the missile 10 toward a point in space other than that at which the target is actually located. Reference to FIG. 2 of the drawings will bring out the basic differ ence between the true Doppler signal return from the target 14 (as picked up by the missile l0) and the false Doppler radiation, or jamming signal, generated on the target 14 and transmitted therefrom to the missile 10 for evasive purposes. The showing of FIG. 2 presents the true Doppler return (on an amplitude versus frequency basis) by the position on the graph of the signal 20. This signal, on the frequency scale, is located at a position designated as f,,+A f this frequency being the original transmitter frequency f of the signal 16 to which has been added the true Doppler frequency 43]]. On the other hand, the energy generated on the target I4 and radiated therefrom may be represented by the signal 22. This signal, to which the spurious Doppler frequency Ajl has been added. is shown as being higher in frequency than the true Doppler signal 20. When the energy represented by the signal 22 is received by the missile 10, the tracking apparatus thereon will direct the missile toward a point in space other than that at which target 14 is located, and hence the objective of the jamming action has been achieved and destruction of the target 14 prevented.

In FIG. 3 is shown one form of apparatus in accordance with the present invention that may be employed upon the target-seeking missile 10 in order to control the latter regardless of any jamming signal that the target I4 may transmit to decoy the missile I0 from its objective. This apparatus, includes an antenna 24 designed to receive the signal 18 and feed the latter to a standard mixer 26 which also receives the output of a local oscillator 28in the usual fashion. In common with conventional networks of the type being described, there is present an intermediate frequency amplifier 30 arranged to amplify the signal which results from beating together the output of the local oscillator 28 and the signal received by antenna 24.

A feature of the present invention resides in the provision of a pair of discriminators A and B which are also identified in the drawing by the reference numerals 32 and 34, respectivel Each of these two discriminators, 32 and 34 receives the output of the IF amplifier 30, as shown. The discriminator A is designed to respond to a range rate R, which may be designated as the rate of change of the true Doppler signal f -i-A f The design of the discriminator 32 is such, however, that it will also develop an output in response to the reception thereby of a false doppler signal f 'l'Af 'l'Afg- The discriminator 34 (or discriminator B) is arranged to respond to the range rate R divided by a factor X thereby making its response either faster or slower than R depending upon the value assigned to X In other words, the respective circuits of discriminator A and discriminator B operate with different response rates.

As also shown in FIG. 3 of the drawings, the respective outputs of discriminators A and B are applied to a memory (or logic) circuit 36. The function of the memory circuit 36 is to control the circuit status of pair of gates 38 and 40, the former also receiving the output of discriminator A as shown and the latter receiving the output of discriminator B. According to the circuit status of gates 38 and 40, either the output of discriminator A or that of discriminator B is connected to an output circuit (not shown) over conductor 42.

The operation of the circuit of FIG. 3 may be described as follows: lnitially, the circuit will track the target by demodulating the true Doppler signal therefrom, which varies at the range rate R, this signal passing through discriminator A through gate 38 to the outputcircuit over conductor 42. At the same time, information as to this range rate R is stored in the memory circuit 36 which also receives the output of discriminator A. There will be also an output from discriminator B under conditions when no jamming signal is present. and this output is likewise applied to the memory circuit 36. The latter therefore recognizes a condition of balance. or in other words a given ratio. between the respective outputs of discriminators A and B under conditions when only the true Doppler signal is being received from the target to close the gate 40.

Now, however, under conditions when a false Doppler signal of different frequency Af, is present in the output of the IF amplifier 30, then the output of discriminator A, as applied to the memory circuit 36, will no longer bear the same ratio to the output of discriminator B as was previously the case. This is true because, in the example illustrated in FIG. 2, the spurious doppler frequency f,,+Af,+Aj is higher than the true doppler signal and hence the response of discriminator 34 (R/Xn), being slower than the response of discriminator 32, the former does not develop an appreciable output in response to the reception of this higher-frequency false doppler. On the other hand, discriminator 32 does develop a certain output from both received signals. The ratio between the respective outputs of the discriminators as seen by the memory circuit 36, however, is not the same as that which existed prior to the time the false doppler signal was received. The memory circuit 36 will recognize this change in the two signals received thereby from the discriminators, and will close the gate 38 so that the output of discriminator B alone will be applied through gate 40 over conductor 42 to the output circuit. This condition will continue as long as there is an unbalance between the original outputs of discriminators A and B. In other words, the circuit of FIG. 3 in effect refuses to track with the false Doppler signal and instead continues to track in the manner which prevailed before the false Doppler signal was received. When the jamming transmission terminates, a state of balance is again achieved between the respective outputs of the two discriminators, and the memory circuit 36 will recognize this condition to open gate 38 and close gate 40. In other words, the network is again conditioned so that the missile tracks with discriminator A as was originally the case. It may therefore be said that in effect the network of FIG. 3 holds the real target signal even under the duress of jamming energy being simultaneously received, and utilizes this stored information during the time when reception of a true Doppler signal return is impossible because of the interference caused by the jamming transmission.

In FIG. 4 of the drawings is illustrated one manner in which the circuit of FIG. 3 may be modified in order to prevent the true Doppler signal from being blocked or saturated by the jamming energy. In this circuit of FIG. 4, there is disclosed an automatic-gain-control circuit 44 which receives the respective outputs of discriminators A and B and also a signal from the memory or logic circuit 36. Consequently, an AGC voltage is developed in both discriminators A and B, but this voltage is under the control of the memory circuit 36 so that no AGC voltage is developed from the jamming energy but only from the true Doppler signal according to whether discriminator A or discriminator B is tracking the target at any particular instant of time. Thus, only the true Doppler signal develops the AGC voltage instead of the latter being established on the basis of much higheramplitude jamming radiation.

In FIG. 5 of the drawings is shown a modification of the circuit of FIG. 4 in which the response of discriminator B, instead of being predetermined as in the case of the circuits of FIGS. 3 and 4, is instead of a variable nature and is capable of responding to a change in range rate. This circuit for controlling the factor X is identified in FIG. 5 of the drawings by the reference numeral 46. When the circuit 46 is employed, the factor X can be made to respond to changes in range rate, or, in other words, the response of the discriminator B automaticaily adjusts to its environment and thereby enhances the counter-countermeasures protection afforded by the disclosed network.

Obviously many modifications and variations of the present invention are possible in the light of the above teachings. It is therefore to be understood that within the scope of the appended claims the invention may be practiced otherwise than as specifically described.

We claim:

I. In signal processing apparatus of the type wherein a signal is radiated from a source toward a moving target, with the energy reflected therefrom then being received at a third point as a Doppler signal and utilized to provide an indication as to the location of such target, the latter being provided with means for receiving the radiated signal from said source and transmitting energy to said third point in the form of a false Doppler signal which differs in character from the true Doppler signal and is intended to provide an erroneous indication as to the actual location of said target, the combination of:

means at said third point for receiving the energy arriving from said target;

a first discriminator having a predetermined response rate;

a second discriminator having a response rate different from that of said first discriminator;

means for applying to both of said discriminators the energy received from said target;

a pair of gating units connected to the respective outputs of said discriminators; and

means, including a memory network connected to both said discriminators, for controlling the circuit status of said gating units in accordance with the reception by said two discriminators of either a true Doppler signal or a false Doppler signal from said target,

whereby the indication provided as to the location of said target will at all times be accurately representative thereof.

2. The combination of claim I, wherein said first discriminator is responsive to the quantity where is the rate at which the range of the target varies with respect to the point at which the Dop' pler signals are received.

3. The combination of claim 2, wherein said second discriminator is responsive to the quantity where R is the rate at which the range of the target varies with respect to the point at which the Doppler signals are received, and X is arbitrarily chosen to yield a response time for said second discriminator which is either faster or slower than the response time of said first discriminator.

4. The combination of claim 3, in which said memory network is designed to close one of said gating units and open the other when the respective outputs of said two discriminators are balanced, and to open said one gating unit and close said other unit when the respective outputs of the said two discriminators are in condition of unbalance.

5. The combination of claim 4 in which the means at said third point for receiving the energy arriving from said target includes an intermediate-frequency amplifier, further comprising an automatic gain control circuit connected to said intermediate-frequency amplifier, said automatic gain control circuit receiving a portion of the output of each of said discriminators; and

a connection between said automatic gain control circuit and said memory network,

whereby the output of said memory network is effective to control the operation of said automatic gain control circuit according to whether the former recognizes a balance or an unbalanced condition between said two discriminators.

6. The combination of claim 4, further comprising means connected to said memory network for deriving a control variation representative of the difference between and am. and

means for applying the variation derived by said lastmentioned means to said second discriminator to establish the quantity iz/Rx R and hence determine the response rate of said second discriminator.

7. In a counter-countermeasures system of the type in which a signal is radiated from a missile-carrying aircraft toward a moving target. with the energy reflected therefrom then being received on a missile launched by said aircraft and utilized to guide such missile toward a point of impact on said target. the latter being equipped with means for receiving the radiated signal from said source and transmitting energy to said missile in the form of a false Doppler signal which differs in character from the true Doppler signal and is intended to provide an erroneous indication as to the actual location of said target, thereby decoying said missile away from impact on said target, the combination of:

means on said missile for receiving the energy arriving from said target; a first discriminator having a predetermined response rate; a second discriminator having a response rate different from that of said first discriminator; means for applying to both of said discriminators the energy received from said target; a pair of gating units connected to the respective outputs of said discriminators; and means. including a memory netork connected to both said discriminators, for controlling the circuit status of said gating units in accordance with the receptive by said two discriminators of either a true Doppler signal or a false Doppler signal from said target, whereby said missile will be guided toward a point of impact on said target regardless of the decoy measures undertaken by the latter. 

1. In signal processing apparatus of the type wherein a signal is radiated from a source toward a moving target, with the energy reflected therefrom then being received at a third point as a Doppler signal and utilized to provide an indication as to the location of such target, the latter being provided with means for receiving the radiated signal from said source and transmitting energy to said third point in the form of a false Doppler signal which differs in character from the true Doppler signal and is intended to provide an erroneous indication as to the actual location of said target, the combination of: means at said third point for receiving the energy arriving from said target; a first discriminator having a predetermined response rate; a second discriminator having a response rate different from that of said first discriminator; means for applying to both of said discriminators the energy received from said target; a pair of gating units connected to the respective outputs of said discriminators; and means, including a memory network connected to both said discriminators, for controlling the circuit status of said gating units in accordance with the reception by said two discriminators of either a true Doppler signal or a false Doppler signal from said target, whereby the indication provided as to the location of said target will at all times be accurately representative thereof.
 2. The combination of claim 1, wherein said first discriminator is responsive to the quantity R where R is the rate at which the range of the target varies with respect to the point at which the Doppler signals are received.
 3. The combination of claim 2, wherein said second discriminator is responsive to the quantity R/XR where R is the rate at which the range of the target varies with respect to the point at which the Doppler signals are received, and X is arbitrarily chosen to yield a response time for said second discriminator which is either faster or slower than the response time of said first discriminator.
 4. The combination of claim 3, in which said memory network is designed to close one of said gating units and open the other when the respective outputs of said two discriminators are balanced, and to open said one gating unit and close said other unit when the respective outputs of the said two discriminators are in condition of unbalance.
 5. The combination of claim 4 in which the means at said third point for receiving the energy arriving from said target includes an intermediate-frequency amplifier, further comprising an automatic gain control circuit connected to said intermediate-frequency amplifier, said automatic gain control circuit receiving a portion of the output of each of said discriminators; and a connection between said automatic gain control circuit and said memory network, whereby the output of said memory network is effective to control the operation of said automatic gain control circuit according to whether the former recognizes a balance or an unbalanced condition between said two discriminators.
 6. The combination of claim 4, further comprising means connected to said memory network for deriving a control variation representative of the difference between R and R/X, and R means for applying the variation derived by said last-mentioned means to said second discriminator to establish the quantity R/X R and hence determine the response rate of said second discriminator.
 7. In a counter-countermeasures system of the type in which a signal is radiated from a missile-carrying aircraft toward a moving target, with the energy reflected therefrom then being received on a missile launched by said aIrcraft and utilized to guide such missile toward a point of impact on said target, the latter being equipped with means for receiving the radiated signal from said source and transmitting energy to said missile in the form of a false Doppler signal which differs in character from the true Doppler signal and is intended to provide an erroneous indication as to the actual location of said target, thereby decoying said missile away from impact on said target, the combination of: means on said missile for receiving the energy arriving from said target; a first discriminator having a predetermined response rate; a second discriminator having a response rate different from that of said first discriminator; means for applying to both of said discriminators the energy received from said target; a pair of gating units connected to the respective outputs of said discriminators; and means, including a memory netork connected to both said discriminators, for controlling the circuit status of said gating units in accordance with the receptive by said two discriminators of either a true Doppler signal or a false Doppler signal from said target, whereby said missile will be guided toward a point of impact on said target regardless of the decoy measures undertaken by the latter. 